Intervertebral spacer device utilizing a spirally slotted belleville washer having radially spaced concentric grooves

ABSTRACT

An intervertebral spacer device having a pair of opposing plates for seating against opposing vertebral bone surfaces, separated by at least one spring mechanism. The preferred spring mechanism is at least one spirally slotted belleville washer having radially spaced concentric grooves. In a preferred embodiment there is a single such belleville washer which is modified to mount onto a ball-shaped head. The lower plate of this embodiment includes a post extending upwardly from the inner surface of the plate, the post including a ball-shaped head. The modified belleville washer can be rotatably mounted to the head such that the wider portion of the washer seats against the upper plate.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 09/789,936, filed Feb. 15, 2001, which is fullyincorporated herein by reference.

FIELD OF THE INVENTION

[0002] This invention relates generally to a spinal implant assembly forimplantation into the intervertebral space between adjacent vertebralbones to simultaneously provide stabilization and continued flexibilityand proper anatomical motion, and more specifically to such a devicewhich utilizes a spirally slotted belleville washer, having radiallyspaced concentric grooves, as a restoring force generating element.

BACKGROUND OF THE INVENTION

[0003] The bones and connective tissue of an adult human spinal columnconsists of more than 20 discrete bones coupled sequentially to oneanother by a tri-joint complex which consists of an anterior disc andthe two posterior facet joints, the anterior discs of adjacent bonesbeing cushioned by cartilage spacers referred to as intervertebraldiscs. These more than 20 bones are anatomically categorized as beingmembers of one of four classifications: cervical, thoracic, lumbar, orsacral. The cervical portion of the spine, which comprises the top ofthe spine, up to the base of the skull, includes the first 7 vertebrae.The intermediate 12 bones are the thoracic vertebrae, and connect to thelower spine comprising the 5 lumbar vertebrae. The base of the spine isthe sacral bones (including the coccyx). The component bones of thecervical spine are generally smaller than those of the thoracic spine,which are in turn smaller than those of the lumbar region. The sacralregion connects laterally to the pelvis. While the sacral region is anintegral part of the spine, for the purposes of fusion surgeries and forthis disclosure, the word spine shall refer only to the cervical,thoracic, and lumbar regions.

[0004] The spinal column of bones is highly complex in that it includesover twenty bones coupled to one another, housing and protectingcritical elements of the nervous system having innumerable peripheralnerves and circulatory bodies in close proximity. In spite of thesecomplications, the spine is a highly flexible structure, capable of ahigh degree of curvature and twist in nearly every direction.

[0005] Genetic or developmental irregularities, trauma, chronic stress,tumors, and degenerative wear are a few of the causes that can result inspinal pathologies for which surgical intervention may be necessary. Avariety of systems have been disclosed in the art which achieveimmobilization and/or fusion of adjacent bones by implanting artificialassemblies in or on the spinal column. The region of the back whichneeds to be immobilized, as well as the individual variations inanatomy, determine the appropriate surgical protocol and implantationassembly. With respect to the failure of the intervertebral disc, theinterbody fusion cage has generated substantial interest because it canbe implanted laparoscopically into the anterior of the spine, thusreducing operating room time, patient recovery time, and scarification.

[0006] Referring now to FIGS. 1 and 2, in which a side perspective viewof an intervertebral body cage and an anterior perspective view of apost implantation spinal column are shown, respectively, a more completedescription of these devices of the prior art is herein provided. Thesecages 10 generally comprise tubular metal body 12 having an externalsurface threading 14. They are inserted transverse to the axis of thespine 16, into preformed cylindrical holes at the junction of adjacentvertebral bodies (in FIG. 2 the pair of cages 10 are inserted betweenthe fifth lumbar vertebra (L5) and the top of the sacrum (S1). Two cages10 are generally inserted side by side with the external threading 14tapping into the lower surface of the vertebral bone above (L5), and theupper surface of the vertebral bone (S1) below. The cages 10 includeholes 18 through which the adjacent bones are to grow. Additionalmaterial, for example autogenous bone graft materials, may be insertedinto the hollow interior 20 of the cage 10 to incite or accelerate thegrowth of the bone into the cage. End caps (not shown) are oftenutilized to hold the bone graft material within the cage 10.

[0007] These cages of the prior art have enjoyed medical success inpromoting fusion and grossly approximating proper disc height. It is,however, important to note that the fusion of the adjacent bones is anincomplete solution to the underlying pathology as it does not cure theailment, but rather simply masks the pathology under a stabilizingbridge of bone. This bone fusion limits the overall flexibility of thespinal column and artificially constrains the normal motion of thepatient. This constraint can cause collateral injury to the patient'sspine as additional stresses of motion, normally borne by the now-fusedjoint, are transferred onto the nearby facet joints and intervertebraldiscs. It would therefore, be a considerable advance in the art toprovide an implant assembly which does not promote fusion, but, rather,which nearly completely mimics the biomechanical action of the naturaldisc cartilage, thereby permitting continued normal motion and stressdistribution.

[0008] It is, therefore, an object of the present invention to provide anew and novel intervertebral spacer which stabilizes the spine withoutpromoting a bone fusion across the intervertebral space.

[0009] It is further an object of the present invention to provide animplant device which stabilizes the spine while still permitting normalmotion.

[0010] It is further an object of the present invention to provide adevice for implantation into the intervertebral space which does notpromote the abnormal distribution of biomechanical stresses on thepatient's spine.

[0011] Other objects of the present invention not explicitly stated willbe set forth and will be more clearly understood in conjunction with thedescriptions of the preferred embodiments disclosed hereafter.

SUMMARY OF THE INVENTION

[0012] The preceding objects of the invention are achieved by thepresent invention which is a flexible intervertebral spacer devicecomprising a pair of spaced apart base plates, arranged in asubstantially parallel planar alignment (or slightly offset relative toone another in accordance with proper lordotic angulation) and coupledto one another by means of a spring mechanism. In particular, thisspring mechanism provides a strong restoring force when a compressiveload is applied to the plates, and may also permit rotation of the twoplates relative to one another. While there are a wide variety ofembodiments contemplated, a preferred embodiment includes a bellevillewasher utilized as the restoring force providing element, the bellevillewasher being spirally slotted and having radially spaced concentricgrooves.

[0013] More particularly, as the assembly is to be positioned betweenthe facing surfaces of adjacent vertebral bodies, the base plates shouldhave substantially flat external surfaces which seat against theopposing bone surfaces. Inasmuch as these bone surfaces are oftenconcave, it is anticipated that the opposing plates may be convex inaccordance with the average topology of the spinal anatomy. In addition,the plates are to mate with the bone surfaces in such a way as to notrotate relative thereto. (The plates rotate relative to one another, butnot with respect to the bone surfaces to which they are each in contactwith.) In order to prevent rotation of a plate relative to the bone, theupper and lower plates can include a porous coating into which the boneof the vertebral body can grow. (Note that this limited fusion of thebone to the base plate does not extend across the intervertebral space.)

[0014] In some embodiments (not in the preferred embodiment), betweenthe base plates, on the exterior of the device, there is included acircumferential wall which is resilient and which simply preventsvessels and tissues from entering within the interior of the device.This resilient wall may comprise a porous fabric or a semi-impermeableelastomeric material. Suitable tissue compatible materials meeting thesimple mechanical requirements of flexibility and durability areprevalent in a number of medical fields including cardiovascularmedicine, wherein such materials are utilized for venous and arterialwall repair, or for use with artificial valve replacements.Alternatively, suitable plastic materials are utilized in the surgicalrepair of gross damage to muscles and organs. Still further materialsthat could be utilized herein may be found in the field of orthopedic inconjunction with ligament and tendon repair. It is anticipated thatfuture developments in this area will produce materials that arecompatible for use with this invention, the breadth of which shall notbe limited by the choice of such a material.

[0015] As introduced above, the internal structure of the presentinvention comprises a spring member, which provides a restoring forcewhen compressed. More particularly, it is desirable that the restoringforces be directed outward against the opposing plates, when acompressive load is applied to the plates. In addition, in certainembodiments, it is necessary that the restoring force providingsubassembly not substantially interfere with the rotation of theopposing plates relative to one another. In the preferred embodiment,the spring subassembly is configured to allow rotation of the platesrelative to one another. In other embodiments, the spring subassemblycan be configured to either allow rotation of the plates, or preventrotation of the plates (through the tightening of a set screw asdiscussed below). As further mentioned above, the force restoring membercomprises at least one belleville washer.

[0016] Belleville washers are washers which are generally bowed in theradial direction. Specifically, they have a radial convexity (i.e., theheight of the washer is not linearly related to the radial distance, butmay, for example, be parabolic in shape). The restoring force of abelleville washer is proportional to the elastic properties of thematerial. In addition, the magnitude of the compressive load support andthe restoring force provided by the belleville washer may be modified byproviding slots and/or grooves in the washer. In the preferredembodiment of the present invention, the belleville washer utilized asthe force restoring member is spirally slotted, with the slotsinitiating on the periphery of the washer and extending along arcs whichare generally radially inwardly directed a distance toward the center ofthe bowed disc, and has radially uniformly spaced concentric grooves ofuniform width and depth.

[0017] As a compressive load is applied to a belleville washer, theforces are directed into a hoop stress which tends to radially expandthe washer. This hoop stress is counterbalanced by the material strengthof the washer, and the strain of the material causes a deflection in theheight of the washer. Stated equivalently, a belleville washer respondsto a compressive load by deflecting compressively, but provides arestoring force which is proportional to the elastic modulus of thematerial in a hoop stressed condition. With slots and/or radially spacedconcentric grooves formed in the washer, it expands and restores itselffar more elastically than a solid washer.

[0018] In general, the belleville washer is one of the strongestconfigurations for a spring, and is highly suitable for use as arestoring force providing subassembly for use in an intervertebralspacer element which must endure considerable cyclical loading in anactive human adult.

[0019] In the preferred embodiment of the present invention, a singlemodified belleville washer, which is of the slotted variety and hasradially spaced concentric grooves as described above, is utilized inconjunction with a ball-shaped post on which it is free to rotatethrough a range of angles (thus permitting the plates to rotate relativeto one another through a corresponding range of angles). Moreparticularly, this embodiment comprises a pair of spaced apart baseplates, one of which is simply a disc shaped member (preferably shapedto match the end of an intervertebral disc) having an external face(having the porous coating discussed above) and an internal face havingan annular retaining wall (the purpose of which will be discussedbelow). The other of the plates is similarly shaped, having an exteriorface with a porous coating, but further includes on its internal face acentral post portion which rises out of the internal face at a nearlyperpendicular angle. The top of this post portion includes a ball-shapedknob. The knob includes a central threaded axial bore which receives asmall set screw. Prior to the insertion of the set screw, theball-shaped head of the post can deflect radially inward (so that theball-shaped knob contracts). The insertion of the set screw eliminatesthe capacity for this deflection.

[0020] As introduced above, a modified and spirally slotted bellevillewasher having radially spaced concentric grooves is mounted to thisball-shaped knob in such a way that it may rotate freely through a rangeof angles equivalent to the fraction of normal human spine rotation (tomimic normal disc rotation). The belleville washer of this design ismodified by including an enlarged inner circumferential portion (at thecenter of the washer) which accommodates the ball-shaped portion of thepost. More particularly, the enlarged portion of the modified bellevillewasher includes a curvate volume having a substantially constant radiusof curvature which is also substantially equivalent to the radius of theball-shaped head of the post. The deflectability of the ball-shaped headof the post, prior to the insertion of the set screw, permits the headto be inserted into the interior volume at the center of the bellevillewasher. Subsequent introduction of the set screw into the axial bore ofthe post prevents the ball-shaped head from deflecting. Thereby, thewasher can be secured to the ball-shaped head so that it can rotatethereon through a range of proper lordotic angles (in some embodiments,a tightening of the set screw locks the washer on the ball-shaped headat one of the lordotic angles).

[0021] This assembly provides ample spring-like performance with respectto axial compressive loads, as well as long cycle life to mimic theaxial biomechanical performance of the normal human intervertebral disc.The spiral slots and radially spaced concentric grooves of thebelleville washer allow the washer to expand radially as the slots andgrooves widen under the load, only to spring back into its undeflectedshape upon the unloading of the spring. As the washer compresses anddecompresses, the annual retaining wall maintains the wide end of thewasher within a prescribed boundary on the internal face of the baseplate which it contacts, and an annular retaining ring maintains thewide end of the washer against the internal face.

[0022] Finally, inasmuch as the human body has a tendency to producefibrous tissues in perceived voids, such as may be found within theinterior of the present invention, and such fibrous tissues mayinterfere with the stable and/or predicted functioning of the device,some embodiments of the present invention (although not the preferredembodiment) will be filled with a highly resilient elastomeric material.The material itself should be highly biologically inert, and should notsubstantially interfere with the restoring forces provided by thespringlike mechanisms therein. Suitable materials may includehydrophilic monomers such as are used in contact lenses. Alternativematerials include silicone jellies and collagens such as have been usedin cosmetic applications. As with the exterior circumferential wall,which was described above as having a variety of suitable alternativematerials, it is anticipated that future research will producealternatives to the materials described herein, and that the futureexistence of such materials which may be used in conjunction with thepresent invention shall not limit the breadth thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a side perspective view of an interbody fusion device ofthe prior art.

[0024]FIG. 2 is a front view of the anterior portion of the lumbo-sacralregion of a human spine, into which a pair of interbody fusion devicesof the type shown in FIG. 1 have been implanted.

[0025]FIGS. 3a and 3 b are side cross-section views of the upper andlower opposing plates of the preferred embodiment of the presentinvention.

[0026]FIGS. 4a and 4 b are top and side cross-section view of abelleville washer having radially uniformly spaced concentric grooves ofuniform width and depth and spiral slots, for use in a preferredembodiment of the present invention.

[0027]FIGS. 5a-5 c are top and side cross-section views of a bellevillewasher having radially non-uniformly spaced concentric grooves ofvarying width and depth and spiral slots, for use in an alternateembodiment of the present invention.

[0028]FIG. 6a is a top view of the upper plate of FIG. 3a, with thebelleville washer of FIGS. 4a and 4 b fitted within a retaining wall anda retaining ring of the upper plate.

[0029]FIG. 6b is a top view of the lower plate of FIG. 3b.

[0030]FIG. 7 is a side cross-section view of the preferred embodiment ofthe present invention, which utilizes a belleville washer of the typeshown in FIGS. 4a and 4 b, showing the plates of FIGS. 6a and 6 bassembled together.

[0031]FIG. 8a is a top view of the upper plate of FIG. 3a, with thebelleville washer of FIGS. 5a-5 c fitted within a retaining wall and aretaining ring of the upper plate.

[0032]FIG. 8b is a top view of the lower plate of FIG. 3b.

[0033]FIG. 9 is a cross-section view of an alternate embodiment of thepresent invention, which utilizes a belleville washer of the type shownin FIGS. 5a-5 c, showing the plates of FIGS. 8a and 8 b assembledtogether.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0034] While the present invention will be described more fullyhereinafter with reference to the accompanying drawings, in whichparticular embodiments and methods of implantation are shown, it is tobe understood at the outset that persons skilled in the art may modifythe invention herein described while achieving the functions and resultsof this invention. Accordingly, the descriptions which follow are to beunderstood as illustrative and exemplary of specific structures, aspectsand features within the broad scope of the present invention and not aslimiting of such broad scope. Like numbers refer to similar features oflike elements throughout.

[0035] Referring now to FIGS. 3a and 3 b, side cross-section views ofupper and lower plate members 100, 200 of the preferred embodiment ofthe present invention are shown. As the device is designed to bepositioned between the facing surfaces of adjacent vertebral bodies, theplates include substantially flat external face portions 102, 202 whichseat against the opposing bone surfaces. In addition, the plates are tomate with the bone surfaces in such a Ail way as to not rotate relativethereto. It is, therefore, preferred that the external faces of theplates include a porous coating 104, 204 into which the bone of thevertebral body can grow. (Note that this limited fusion of the bone tothe base plate does not extend across the intervertebral space.) A hole(not shown) can be provided in the upper plate such that the interior ofthe device may be readily accessed if a need should arise.

[0036] The upper plate 100 includes an internal face 103 that includesan annular retaining wall 108 and an annular retaining ring 109. Thelower plate 200 includes an internal face 203 that includes a centralpost member 201 which rises out of the internal face 203 at a nearlyperpendicular angle. The top of this post member 201 includes aball-shaped head 207. The head 207 includes a series of slots whichrender it compressible and expandable in correspondence with a radialpressure (or a radial component of a pressure applied thereto). The head207 includes a central threaded axial bore 209 which extends down thepost 201. This threaded bore 209 is designed to receive a set screw 205.Prior to the insertion of the set screw 205, the ball-shaped head 207 ofthe post 201 can deflect radially inward because of the slots (so thatthe ball-shaped head contracts). The insertion of the set screw 205eliminates the capacity for this deflection.

[0037] Referring now to FIGS. 4a and 4 b, a spirally slotted bellevillewasher 130 having radially spaced concentric grooves is provided in topand side cross-section views. The belleville washer 130 is a restoringforce providing device which comprises a circular shape, having acentral opening 132, and which is radially arched in shape. Thebelleville washer 130 has a radial convexity 134 (i.e., the height ofthe washer 130 is not linearly related to the radial distance, but may,for example, be parabolic in shape). The restoring force of thebelleville washer 130 is proportional to the elastic properties of thematerial.

[0038] The belleville washer 130 comprises a series of spiral slots 131formed therein. The slots 131 extend from the outer edge of thebelleville washer, inward along arcs generally directed toward thecenter of the element. The slots 131 do not extend fully to the centerof the element. In preferred embodiments, the slots may extend anywherefrom a quarter to three quarters of the overall radius of the washer,depending upon the requirements of the patient, and the anatomicalrequirements of the device.

[0039] The belleville washer 130 further comprises a series of grooves133 formed therein. The grooves 133 are concentric and radially spacedfrom the outer edge of the belleville washer toward the center of theelement. In the preferred embodiment shown in FIGS. 4a and 4 b, thewidth 135 of each groove 133 is uniform along the length of the groove133. Further in the preferred embodiment, the depth 137 of each groove133 is uniform along the length of the groove 133. Further in thepreferred embodiment, each groove 133 has a different widthconfiguration and a different depth configuration than each other groove133. More specifically, in the preferred embodiment, the width dimensionand the depth dimension both vary from groove to groove, each increasingincrementally from groove to adjacent groove with increasing distancefrom the center of the washer 130. Stated alternatively, grooves thatare relatively more narrow and more shallow than the other grooves arecloser to the center of the washer, whereas grooves that are relativelywider and deeper than the other grooves are closer to the outer edge ofthe washer. This is illustrated by example in FIGS. 4a and 4 b, whichshow three concentric grooves 133 a-c, with the outermost groove 133 cbeing deeper and wider than groove 133 b, which is in turn deeper andwider than groove 133 a. Further in the preferred embodiment, the radialspacing of the grooves is uniform.

[0040] It should be understood that in other embodiments, one or both ofthe depth and the width of each groove can be (1) increasing along thelength of the groove, (2) decreasing along the length of the groove, or(3) varied along the length of each groove, either randomly or accordingto a pattern. Moreover, in other embodiments, it can be the case thateach groove is not formed similarly to one or more other grooves, withor without respect to width and depth dimensions, but rather one or moregrooves are formed in any of the above-mentioned fashions, while one ormore other grooves are formed in another of the above-mentioned fashionsor other fashions. Also, in other embodiments, it can be the case thatthe radial distance between the grooves is not the same, but rather thespacing increases the closer the space is to the outer edge of thewasher, decreases the closer the space is to the outer edge of thewasher, or varies either randomly or according to a pattern. Also, whilethe grooves of the preferred embodiment and the illustrated alternateembodiment have lengths that form closed loops, it should be noted thatin other embodiments, the concentric grooves can have lengths that formopen loops or arcs; for example, a two concentric grooves forming openloops or arcs can be used in place of a single concentric groove forminga closed loop. It should be clear that any concentric groove pattern canbe implemented without departing from the scope of the presentinvention. To illustrate an alternate embodiment showing an alternateradially spaced concentric groove pattern, FIGS. 5a-5 c show abelleville washer 130 having radially spaced concentric grooves 133 intop and side cross-section views, with each groove 133 having a widthand a depth each varying along the length of the groove 133, with eachgroove 133 being formed differently than at least one other groove 133,with the radial spacing of the grooves 133 being varied, and with bothclosed loops and open loops or arcs being used. In this alternateembodiment, the difference between the grooves 133 is characterized inthat the wider and deeper portion of any particular groove 133 is on adifferent side of the washer 130 than the wider and deeper portion of atleast one other groove 133.

[0041] As a compressive load is applied to the belleville washer 130 ofthe present invention, the forces are directed into a hoop stress whichtends to radially expand the washer. This hoop stress is counterbalancedby the material strength of the washer, and the force necessary to widenthe spiral slots 131 and the radially spaced concentric grooves 133along with the strain of the material causes a deflection in the heightof the washer. Stated equivalently, the belleville washer 130 respondsto a compressive load by deflecting compressively; the spiral slotsand/or radially spaced concentric grooves cause the washer to furtherrespond to the load by spreading as the slots and/or the grooves in thewasher expand under the load. The spring, therefore, provides arestoring force which is proportional to the elastic modulus of thematerial in a hoop stressed condition.

[0042] More particularly, the central opening 132 of the bellevillewasher is enlarged. This central opening 132 includes a curvate volume233 for receiving therein the ball-shaped head 207 of the post 201 ofthe lower plate 200 described above. More particularly, the curvatevolume 233 has a substantially constant radius of curvature which isalso substantially equivalent to the radius of the ball-shaped head 207of the post 201. In this embodiment, the spiral slots 131 do not extendall the way to the central opening 132, and approach the opening only asfar as the material strength of the washer can handle withoutplastically deforming under the expected anatomical loading. Preferably,the center of the washer is flat; therefore, the central opening 132 canbe formed from flat edges. It should be understood that this is notrequired, but rather is preferred.

[0043] Referring now to FIG. 6a, a top view of the upper plate 100 ofFIG. 3a, with the spirally slotted and concentrically grooved bellevillewasher 130 of FIGS. 4a and 4 b fitted within a retaining wall 108 and aretaining ring 109 of the upper plate 100, is shown. The diameter of theretaining wall 108 is preferably slightly wider than the diameter of theundeflected belleville washer 130 such that the loading thereof canresult in an unrestrained radial deflection of the washer 130. FIG. 6bshows a top view of the lower plate 200 of FIG. 3b.

[0044]FIG. 7 shows the fully assembled preferred embodiment of thepresent invention. The spirally slotted and radially grooved bellevillewasher 130 of FIGS. 4a and 4 b is placed with its wide end against thetop plate 100 within the annular retaining wall 108 as shown in FIG. 6b.The annular retaining ring 109 is provided to hold the belleville washer130 against the internal face 103 of the upper plate 100 within theretaining wall 108. The post 201 of the lower plate 200 is fitted intothe central opening 132 of the belleville washer 130 (the deflectabilityof the ball-shaped head 207 of the post 201, prior to the insertion ofthe set screw 205, permits the head 207 to be inserted into the interiorvolume 233 at the center of the belleville washer 130. Subsequentintroduction of the set screw 205 into the axial bore 209 of the post201 eliminates the deflectability of the head 207 so that the washer 130cannot be readily removed therefrom, but can still rotate thereon. Insome embodiments (not in this preferred embodiment), the post head 207can be locked tightly within the central volume 233 of the bellevillewasher 130 by the tightening of the set screw 205, to prevent anyrotation of the plates 100, 200. Compressive loading of the assemblycauses the washer 130 to deflect (with the spiral slots and the radiallyspaced concentric grooves enhancing the deflection) so that the wide endradially expands while being maintained centrally against the upperplate 100 by the retaining wall 108 and the retaining ring 109. When theload is removed, the washer 130 springs back to its original shape.

[0045] Referring now to FIG. 8a, a top view of the upper plate 100 ofFIG. 3a, with the spirally slotted and concentrically grooved bellevillewasher 130 of FIGS. 5a-5 c fitted within a retaining wall 108 and aretaining ring 109 of the upper plate 100, is shown. The diameter of theretaining wall 108 is preferably slightly wider than the diameter of theundeflected belleville washer 130 such that the loading thereof canresult in an unrestrained radial deflection of the washer 130. FIG. 8bshows a top view of the lower plate 200 of FIG. 3b.

[0046]FIG. 9 shows a fully assembled alternate embodiment of the presentinvention. The spirally slotted and concentrically grooved bellevillewasher 130 of FIGS. 5a-5 c is placed with its wide end against the topplate 100 within the annular retaining wall 108 as shown in FIG. 6b. Theannular retaining ring 109 is provided to hold the belleville washer 130against the internal face 103 of the upper plate 100 within theretaining wall 108. The post 201 of the lower plate 200 is fitted intothe central opening 132 of the belleville washer 130 (the deflectabilityof the ball-shaped head 207 of the post 201, prior to the insertion ofthe set screw 205, permits the head 207 to be inserted into the interiorvolume 233 at the center of the belleville washer 130, and the washer130 to be rotated into the desired angulation; subsequent introductionof the set screw 205 into the axial bore 209 of the post 201 eliminatesthe deflectability of the head 207 so that the washer 130 cannot bereadily removed therefrom, but can still rotate thereon.). The post head207 can be locked tightly within the central volume 233 of thebelleville washer 130 by the tightening of the set screw 205, to preventany rotation of the plates 100, 200. Compressive loading of the assemblycauses the washer 130 to deflect (with the spiral slots and radiallyspaced concentric grooves enhancing the deflection) so that the wide endradially expands while being maintained centrally against the upperplate 100 by the retaining wall 108 and the retaining ring 109. When theload is removed, the washer 130 springs back to its original shape.

[0047] Inasmuch as the human body has a tendency to produce fibroustissues in perceived voids, such as may be found within the interior ofthe present invention, and such fibrous tissues may interfere with thestable and/or predicted functioning of the device, some embodiments ofthe present invention (although not the preferred embodiment) will befilled with a highly resilient and biologically inert elastomericmaterial. Suitable materials may include hydrophilic monomers such asare used in contact lenses. Alternative materials include siliconejellies and collagens such as have been used in cosmetic applications.

[0048] While there has been described and illustrated specificembodiments of an intervertebral spacer device, it will be apparent tothose skilled in the art that variations and modifications are possiblewithout deviating from the broad spirit and principle of the presentinvention. The invention, therefore, shall not be limited to thespecific embodiments discussed herein.

We claim:
 1. An intervertebral spacer device comprising: first andsecond plate members, each having first and second plate surfacesthereof, said plates being disposed in a spaced apart relationship suchthat first ones of said plate surfaces oppose one another, and secondones of said plate surfaces face in opposite directions; and at leastone restoring force providing element disposed between said firstsurfaces of said first and second plate members, and disposed such thata compressive load applied to said second surfaces of said first andsecond plate members is counteracted by said at least one restoringforce providing element, said at least one restoring force providingelement including at least one belleville washer having at least oneradially spaced concentric groove and at least one spiral slot.
 2. Thedevice as set forth in claim 1, further comprising disposed on at leastone of said first and second plate members a retaining wall in whichsaid at least one restoring force providing element is contained.
 3. Thedevice as set forth in claim 1, wherein said at least one restoringforce providing element includes a belleville washer having a narrow endand a wide end and being oriented such that said wide end is in contactwith said first surface of one of said first and second plate members.4. The device as set forth in claim 1, wherein said at least oneradially spaced concentric groove in said at least one belleville washercomprises a plurality of radially spaced concentric grooves, and whereinat least one of said plurality of radially spaced concentric grooves insaid at least one belleville washer has a length.
 5. The device as setforth in claim 4, wherein said at least one of said plurality ofradially spaced concentric grooves in said at least one bellevillewasher has a depth and a width, and at least one of said width and saiddepth is uniform along said length.
 6. The device as set forth in claim4, wherein said at least one of said plurality of radially spacedconcentric grooves in said at least one belleville washer has a depthand a width, and at least one of said width and said depth varies alongsaid length.
 7. The device as set forth in claim 4, wherein said lengthforms a closed loop.
 8. The device as set forth in claim 4, wherein saidlength forms an open loop.
 9. The device as set forth in claim 4,wherein the radial spacing of said plurality of radially spacedconcentric grooves is uniform.
 10. The device as set forth in claim 4,wherein the radial spacing of said plurality of radially spacedconcentric grooves is non-uniform.
 11. The device as set forth in claim4, wherein each of said plurality of radially spaced concentric groovesin said at least one belleville washer has a respective length, arespective depth along said respective length, and a respective widthalong said respective length, and wherein at least one of said depths isdifferent than at least one other of said depths, and wherein at leastone of said widths is different than at least one other of said widths.12. The device as set forth in claim 11, wherein each of said pluralityof radially spaced concentric grooves in said at least one bellevillewasher is at a respective distance from an outer edge of said at leastone belleville washer, wherein said depths increase incrementally withdecreasing said distances, and said widths increase incrementally withdecreasing said distances.
 13. The device as set forth in claim 1,wherein said at least one spiral slot in said at least one bellevillewasher comprises a plurality of spaced apart spiral slots, each of whichextends from a locus on a peripheral edge of said at least onebelleville washer to a locus which is radially in from said peripheraledge.
 14. The device as set forth in claim 1, wherein one of said firstand second plate members further comprises a post structure extendingoutwardly from said first surface of said one of said first and secondplate members, and which post structure includes a ball-shaped head. 15.The device as set forth in claim 14, wherein said post structure furtherincludes a threaded bore which extends axially from said ball-shapedhead toward said first surface of said one of said first and secondplate members, and which bore receives therein a threaded set screw suchthat prior to an insertion of the set screw therein, said bore permitsthe ball-shaped head to compress radially inwardly, and such that afterthe insertion of said set screw said ball-shaped head is not readilyradially compressible.
 16. The device as set forth in claim 15, whereinsaid at least one belleville washer further comprises a central openingwhich includes a curvate volume for receiving and holding therein saidball-shaped head.
 17. The device as set forth in claim 16, wherein atightening of said set screw locks said curvate volume with respect tosaid ball-shaped head.
 18. An intervertebral spacer device comprising:first and second plates, each having inner and outer surfaces thereof,said plates being disposed in a spaced apart relationship such that theinner surfaces face toward one another, and the outer surfaces face awayfrom one another, said first plate including a retaining wall extendingoutwardly from said inner surface of said first plate; and a bellevillewasher, having narrow and wide ends thereof, disposed with said wide endcontacting said inner surface of said first plate within said retainingwall, such that a compressive load applied to the outer surfaces of saidplates is counteracted by said belleville washer, said belleville washerincluding at least one radially spaced concentric groove and at leastone spiral slot.
 19. The device as set forth in claim 18, wherein saidat least one spiral slot in said belleville washer comprises a pluralityof spaced apart spiral slots, each of which extends from a locus on aperipheral edge of said belleville washer to a locus which is radiallyin from said peripheral edge.
 20. The device as set forth in claim 18,wherein said at least one radially spaced concentric groove in saidbelleville washer comprises a plurality of radially spaced concentricgrooves, and wherein each of said plurality of radially spacedconcentric grooves in said belleville washer has a respective length, arespective depth along said respective length, and a respective widthalong said respective length, at least one of said respective depth andsaid respective width being uniform along said respective length.